Edit: To make the question more focused, feel free to answer this question as regards your particular period of knowledge. However, I would be most interested in any centuries from, say, 1300 to 1800. Whenever I ask broad questions, I generally hope to receive more specific answers based on the answerer's (or, better yet, answerers') areas of expertise, rather than to have one big answer covering the whole scope of my question.
It is different per country, but I just read an excellent book on the life of the Dutch aristocracy from 400 years ago to today in the Netherlands and they basically went two ways:
Business
Land rental
Some made a fortune in trade and bought themselves into nobility either by 'buying' a title or marrying into an impoverished noble family. They would invest in land when they could afford it.
The 'real' land-owning aristocracy would always look slightly down on these upstarts but they were generally a lot poorer. Renting land to farmers wasn't exactly a goldmine.
As families grew (and often became rather eccentric) the fortunes made could not keep up and the money got divided into smaller and smaller portions requiring more land to be sold off to make up the difference. By last century the aristocracy simply transformed into upper middle class citizens and faded away as the last land was sold off, except for a tiny, mostly powerless, upper class remnant.
The book is 'De levens van Jan Six' by Geert Mak, a famous Dutch historian. It describes the history from the viewpoint of the aristocratic Six family. The title is a bit of a play on words, 'The lives of Jan Six', there were many men named Jan in the family history.